Method of processing electroplated spring steel parts



Patented Oct. 23, 1951 METHOD OF PROCESSING ELECTROPLATED SPRING STEEL PARTS Kenneth B. -Valentine, Waterford Township, Oakland County, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application November 14, 1946, Serial No. 709,776

4 Claims.

This invention has to do with parts made of spring steel, for example lock washers, flat springs, coil spring, etc., and to methods of processing or treating the same to eliminate or minimize stress cracking thereof.

Breakage of electroplated lock washers and other parts made of hardened and .tempered spring steel heretofore has been a matter of rather common occurrence. Failures by stress cracking have been attributed to hydrogen absorbed in the steel during its manufacture and during pickling and electroplating of the steel, since hydrogen is known to have an embrittling efiect on steel. It has been concluded by some that the greater the thickness of the plate, the greater the likelihood of failure by cracking. In some cases the parts. after electroplating, have been heated to attempt to drive off the absorbed hydrogen.- Relatively low temperatures have been used, for example 400 F., to avoid blistering and deterioration of the plate and to avoid softening the spring steel. Such treatment reduces the percentages of failures but it still has been concluded that some failures must be expected regardless of plating procedure or subsequent aging treatment.

I have discovered that there is another factor which must be taken into consideration, namely the hardness of the steel. Parts formed of spring steel are hardened and tempered to have a rather high hardness. For example, lock washer specifications generally call for a hardness of Rockwell C47 to 53. According to my experience a suitable aging heat treatment subsequent to plating will eliminate or control embrittlement in the hardness range of Rockwell C4'7 to 49. I have discovered that in higher hardness ranges, for example, Rockwell C50 to 56, that the critical embrittlement of parts formed of spring steel (springs, both coiled and fiat, lock washers, etc.) can be eliminated by a subcritical annealing prior to the hardening and tempering heat treating. Stress cracking is much more likely to occur with plated parts than with The subcritical annealing treatment prior to the hardening and tempering heat treatment conunplated parts. In many applications an electrosists in heating the spring steel stock or the parts formed therefrom to a temperature Within the range of about 1000 F. to a temperature below the point of austenization for a period of time varying with. section size and the temperature.

7 In general, a'period. of one hour or'more is neceshas proven highly satisfactory. Longer times 7 may be employed, but are unnecessary. 'Best results have been obtained when the parts are cooled slowly after being given the subcritical anneal. However, it is within the scope of the invention in its broader aspects to quench or rapidly cool the steel, or to heat the steel to the hardening temperature without cooling.

Following the subcritical annealing treatment the spring steel stock or the parts formed therefrom are hardened in conventional manner by heating above the critical temperature of the steel (above A03) then cooling at a sufficiently rapid rate as to harden the same. One suitable method is to heat the steel in a neutral salt bath at a temperature of 1500-1600 F. to austenize the same and then quench in a conventional mineral oil heated to a temperature of 140 F. The hardening treatment is then followed by a tempering or drawing heat treatment which also may be in conventional manner. A temperature within the range of 400 to 750 F. is quite suitable, the exact temperature being dependent on the desired hardness of the steel. Where a protective or decorative finish is required the spring steel is electroplated to provide a plating on the order of .0002 to .0005" in thickness. Zinc and cadmium are examples of electrodeposited metal plates. Any conventional zincor cyanide plating baths may be used. Where the spring steel is electroplated with metals such as zinc or cadmium the plated steel is given a subsequent low temperature heat treatment at a temperature of about 400 F. or less. In general a range of 200 to 400 F. may be employed for a time of one hour or more. A time of four hours at a temperature of 400 F. insures practical optimum benefits.

The invention is applicable to all spring steels. In general, the carbon content of spring steels will range from C to .90%. One common steel employed for lock washers is SAE 1060. This lock washer steel is commonly supplied in the form of Keystone section wire. An auto to austenize the samef uenchings difheated. spring steel to harden the'same, tempering said" matic coil winding machine is one form of apparatus which may be employed in forming the wire into lock washers of annular shape.

In order to test lock washers they may be placed between cyanided flat washers on a cyanided bolt and then drawn fiat with a nut. The lock washers are examined periodically for failures while clamped on the bolts.

The following table gives results obtained with fa" lock washers of heavy stock ('SAE 1060 steel) having .0002 zinc plate obtained by means of a plating solution consisting of ounces per gallon of ZnCn, 12 ounces per gallon of NaCn and 14 ounces per gallon of NaOH. A current density of about amperes per square foot was used. The parts were tested .in the fixture above described.

Table sisting of zinc and cadmium, and then heating said spring steel and electrodeposited coating thereon at a temperature of about 400 F. for a time of about four hours.

.3. The method of processing parts formed of hardened and tempered spring steel having a I u Hardness Number lgggllrggr Per cent Treatment Before Plating Treatment After Plating Bo cgy vell Tested After 1 Broken Week Hardened and tempered None 51 200 58 Do heated at 400 F. for 4 hours... 51 210 13 7 heated at 1100 F. for 2 hours, cooled slowly, and then do 51 203 0 0 hardened and tempered.

D None 51 1208 11 6 heated at 400 F. for 4 ]1ours..- .53 200 0 0 do .L 56 187 0 0 Changes may be madein the embodiments of my;invention described herein without departing from the principles and spirit of the invention.

'I claim: I

L-The method of processing parts formed of hardened and tempered spring steel having a Rockwell C hardness of at least which comprises heating the steel at a temperature within the range of 1000" F. to a temperature just below the lower critical range of the steel (A01) for a time within the range of about one to two hours, thereafter heating the spring steel to' a" tempera-- ture within the range-of about 1500. 0.165021 hardened spring steel by heating the same to a temperature within the range of 400 F. to 750 F., electrodepositing onto said hardened and tempered spring steel acoating of the'class con sisting of zinc and cadmium, and then heating said spring steel and electrodeposited coating thereon ata temperaturewithin the rangeof 200 F. to 400 F. ,for a time within the range of one to four hours.

2. The method of processing parts formed of hardened and tempered spring steel having a Rockwell C hardness of at least 50 which comprises heating the steel at a temperature within the range of 1000" F. to a temperature just below the lower critical range of the steel (A01) for a time within the range of about one to two hours, thereafter heating the spring steel to a temperature within the range of about 1500 F. to 1650? F. to austenize the same, quenching said heated spring steel to harden thesame, tempering said hardened spring steel byheating the same 'to"a' temperature within the range of 400 F. to 750 F., electrodepositing onto said hardened and tempered spring steel a coating of theclass conr at a temperature of the class consisting of zinc and cadmium, and then heating said hardened and tempered steel part and electrodeposited coating thereon at a temperature of about400 F. for a time of about four hours.

4. The method of processing lock washers formed of spring steelhaving a Rockwell C hardness after hardening and tempering within the range of 50-56 which comprises heating the spring steel at a temperature of 1100 F. for two hours, slowly cooling the steel, thereafter heating the steel to a temperature within the range of 1500" F te 1650 F. to austenize the same, quenchmgthe steel to harden the same, tempering the hardened'steel by heating to a temperature withiinthe range of 400 F. to 750 F., electrodepositing aeoa'ting of zinc having a thickness-of .0002

to .0005 onto therhardened and tempered steel part, and thereafter heatingthe' plated steel part of 400 F. for four. hours.

KENNETH B. VALENTINE.

REFERENCES CITED r The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of the American Society for Steel Treating, August 1926, pages 237-239.

The Metallography and Heat Treatment of Iron and Steel by Sauveur, 4th edition, 1935,

' pages 292, 293.

The Making, Shaping and Treating of Steel by Camp and Francis, 5th edition, 1940, page 857. 

1. THE METHOD OF PROCESSING PARTS FORMED OF HARDENED AND TEMPERED SPRING STEEL HAVING A ROCKWELL C HARDNESS OF AT LEAST 50 WHICH COMPRISES HEATING THE STEEL AT A TEMPERATURE WITHIN THE RANGE OF 1000* F. TO A TEMPERATURE JUST BELOW THE LOWER CRITICAL RANGE OF THE STEEL (AC1) FOR A TIME WITHIN THE RANGE OF ABOUT ONE TO TWO HOURS, THEREAFTER HEATING THE SPRING STEEL TO A TEMPERATURE WITHIN THE RANGE OF ABOUT 1500* F TO 1650* F. TO AUSTENIZE THE SAME, QUENCHING SAID HEATED SPRING STEEL TO HARDEN THE SAME, TEMPERING SAID HARDENED SPRING STEEL BY HEATING THE SAME TO A TEMPERATURE WITHIN THE RANGE OF 400* F. TO 750* F., ELECTRODEPOSITING ONTO SAID HARDENED AND TEMPERED SPRING A COATING OF THE CLASS CONSISTING OF ZINC AND CADMIUM, AND THEN HEATINGSAID SPRING STEEL AND ELECTRODEPOSITED COATING THEREON AT A TEMPERATURE WITHIN THE RANGE OF 200* F. TO 400* F. FOR A TIME WITHIN THE RANGE OF ONE TO FOUR HOURS. 